SCIENTIFIC BACKGROUND 175 



termed the Valyermo Section, makes the northern 

 boundary of the San Andreas-Puente Belt. 



NATURE OF THE FAULT MOVEMENTS 

 Vertical movements along the Southern Section of 

 the San Andreas Rift have resulted in the production 

 of many of the master physiographic highlands, val- 

 leys and other lineaments of Southern California. To 

 them are due the sharp, rectilinear scarp-lines of the 

 northeast side and through the northeast end of the 

 San Gabriel Highland, the southwest side of the San 

 Bernardino, and the north end of the San Jacinto 

 master highlands, as well as the southwest border of 

 the Mojave Valley and the north end of the great Gulf 

 of California (Colorado) Depression. 



Lateral (horizontal) movements like that which ac- 

 companied the San Francisco earthquake in 1906 may 

 probably also have taken place along this Southern 

 Section of the San Andreas Rift in past geologic times 

 whereby the northeast or San Bernardino Highland 

 side has been moved southeasterly, and the south- 

 west, or San Gabriel side, to have moved northwester- 

 ly. This has been suggested by the relative positions 

 of these two mountain regions as shown on Plate I, 

 and other related conditions. 



POSSIBILITY OF TWO FAULT LINES 

 On the whole I am prone to inquire if the Northern 

 and Southern Sections of the San Andreas Rift may 

 not consist of two ancient and formerly distinct faults, 

 which intersected at the region of the bending south 

 of Maricopa. There are suggestions in the physiog- 

 raphy which might lead one to infer that the more 

 westerly end of the Southern Section may have rela- 



